The present invention relates generally to electrodes for applying/recording electrical signals to/from the neural/neuromuscular system of a person. The present invention more particularly relates to such electrodes which are inserted into the external ear canal of a person.
It is well known that electrical stimulation of thc auditory system of a person can produce complex perceptions of sounds in human subjects. Experiments by Volta occurred in the early 1800's. Volta inserted metal bars into his external ear canals and passed current between the bars and reported the perception of sound. A series of studies was performed in the late 1930's by Jones, Stevens and Lurie which examined the relationship between various parameters of electrical stimulation and the listed auditory perceps. In these experiments, investigators used metal electrodes placed into saline filled external ear canals to deliver electrical current to the auditory system.
More recently, electrodes surgically implanted into the cochlea, are providing a sensation of hearing in profoundly deaf individuals. The development of cochlear implants has prompted significant additional research in the area of electrical stimulation for hearing augmentation.
Means of stimulating the auditory system which are noninvasive, as opposed to a cochlear implant, are desirable for several reasons. A noninvasive electrode system for auditory stimulation could be used as a functional auditory prosthesis, as a tool in the diagnostic evaluation of potential candidates for cochlear implants or to record the electrical signals generated by an auditory system stimulated by other means.
Several investigators have reported the use of electrodes combined with a saline filled ear canal (known as a Bremmer type electrode) to provide a noninvasive means of auditory stimulation. This approach to the stimulation of the auditory system has a major drawback. Current passed into the saline solution spreads rapidly in all directions through the tissues of the ear and head. The same electrical current which activates the auditory nerve can also stimulate cutaneous nerve fibers thereby producing uncomfortable and, possibly, painful sensations.
Existing systems of auditory stimulation have significant disadvantages. Cochlear implants require surgical invasion of the body. Thus, cochlear implants are not practical for evaluation or diagnostic purposes, which involve temporary stimulation or recording. On the other hand, conventional external ear canal stimulation has significant limitations in dynamic range. The electrical current used in external ear canal stimulation must be above an amount to exceed the hearing threshold but must be below an amount which would produce pain within the external ear canal or, possibly, uncomfortable loud sensations.